Controlled-source electromagnetic surveys are an important geophysical tool for evaluating the presence of hydrocarbon-bearing strata within the earth. CSEM surveys typically record the electromagnetic signal induced in the earth by a source (transmitter) and measured at one or more receivers. The behavior of this signal as a function of transmitter location, frequency, and separation (offset) between transmitter and receiver can be diagnostic of rock properties associated with the presence or absence of hydrocarbons. Specifically, CSEM measurements are used to determine the spatially-varying resistivity of the subsurface.
In the marine environment, CSEM data (“MCSEM” data) are typically acquired by towing an electric bipole transmitting antenna 11 among a number of receivers 12 positioned on the seafloor 13 (FIG. 1). The transmitter antenna is typically towed a few tens of meters above the seafloor. The receivers have multiple sensors designed to record one or more different vector components of the electric and/or magnetic fields. Alternative configurations include stationary transmitters on the seafloor or in the water column as well as magnetic transmitter antennae. Most MCSEM surveys are 3D surveys, i.e., more than one source towline is utilized. The transmitting and receiving systems typically operate independently (without any connection), so that receiver data must be synchronized with shipboard measurements of transmitter position by comparing clock times on the receivers to time from a shipboard or GPS (Global Positioning System) standard.
MCSEM data collected in deep water are typically interpreted in the temporal frequency domain, each signal representing the response of the earth to electromagnetic energy at that temporal frequency. In raw data, the strength of each frequency component varies depending on how much energy the transmitter broadcasts and on the receiver sensitivity at that frequency. These effects are typically removed from the data prior to interpretation. FIGS. 2A and 2B depict raw receiver data 21 together with (in FIG. 2B) the transmitter waveform 22 that gave rise to it. FIG. 2A shows examples of received CSEM signals on a time scale of several hours, while FIG. 2B shows the same received signal on a much shorter time scale, comparable to the period, T, of the transmitter waveform. Typical values for T are between 4 and 64 seconds. The transmitter waveform is depicted as a dashed line overlaying the receiver waveform. (The transmitter waveform is shown for reference only; the vertical scale applies only to the receiver signal.)
In practice, the receiver data are converted to temporal frequency by dividing (or “binning”) the recorded time-domain data into time intervals equal to the transmitter waveform period (FIG. 3A) and determining the spectrum (FIG. 3B) within each bin (x1, x2, x3) by standard methods based on the Fourier Transform. The phases of the spectral components are not shown. With each bin is associated a time, typically the Julian date at the center of the bin. Since the transmitter location is known as a function of time, these bins may be interchangeably labeled in several different ways including: by Julian date of the bin center; by transmitter position; by the signed offset distance between source and receiver; or by the cumulative distance traveled by the transmitter relative to some starting point.
The transmitter signal may be a more complex waveform than that depicted in FIGS. 2B and 3A.
MCSEM receivers (FIG. 4) typically include (see U.S. Pat. No. 5,770,945 to Constable):                a power system, e.g. batteries (inside data logger and pressure case 40);        one or more electric-field (E) or magnetic-field (B) antennae (bipoles 41 receive + and − Ex fields, dipoles 42 + and − Ey, coils 43 for Bx and coils 44 for By);        other measuring devices, such as a compass and thermometer (not shown);        electronics packages that begin sensing, digitizing, and storing these measurements at a pre-programmed time (inside case 40);        a means to extract data from the receiver to a shipboard computer after the receiver returns to the surface (not shown);        a weight (e.g., concrete anchor 44) sufficient to cause the receiver to fall to the seafloor;        a mechanism 45 to release the receiver from its weight upon receiving (acoustic release and navigation unit 46) an acoustic signal from a surface vessel (14 in FIG. 1);        glass flotation spheres 47;        strayline float 48; and        various (not shown) hooks, flags, strobe lights, and radio beacons to simplify deployment and recovery of the receiver from a ship at the surface.        
Clearly, other configurations are possible, such as connecting several receivers in a towed array (see, for example, U.S. Pat. No. 4,617,518 to Srnka). The receiver depicted in FIG. 4 is a 4-component (Ex, Ey, Bx, and By) seafloor CSEM receiver. The devices can be configured to record different field types, including vertical electric (Ez) and magnetic (Bz) fields.
In general, the received signals are made up of components both in-phase and out-of-phase with the transmitter signal. The signals are therefore conveniently represented as complex numbers in either rectangular (real-imaginary) or polar (amplitude-phase) form. As shown in FIGS. 5 and 6, both the phase and amplitude of MCSEM data can be indicative of resistive (and potentially hydrocarbon-bearing) strata. Both the phase and amplitude must be accurately determined in order to distinguish signal characteristics associated with hydrocarbons from the much larger portion of the signal that is associated other geologic features of the subsurface. FIG. 5 shows a cross-section view of a typical MCSEM survey. The signal measured in a receiver 12 has contributions from many different paths through the subsurface, including paths associated with resistive (potentially hydrocarbon-bearing) strata such as 51. FIG. 6A shows Electric-field amplitude and FIG. 6B shows the corresponding phase responses that might result from the MCSEM measurements depicted in FIG. 5. The dashed curves show signals in the absence of the resistive unit 51. Signals in the presence of the resistive unit (solid curves) show a larger amplitude, as current is forced back toward the surface, and a delayed phase, due to the longer wavelengths of electromagnetic waves in the resistive unit.
Every CSEM signal frequency, ω, measured in radians per second is associated with a signal period, T=2π/ω, measured in seconds. Any phase value, φ, or phase shift, Δφ, is associated with an equivalent time shift, Δt, by the formulaΔφ=2π(Δt/T).While phase is customarily measured as an angle between 0 and 2π radians, it can be equivalently thought of as a time between 0 and T seconds.
There are currently several approaches to analyzing MCSEM data. The most robust is full 3D inversion, which directly produces a 3D resistivity volume of the subsurface consistent with specified parameters, generally amplitude and phase of one or more measured components of the electric field at one or more frequencies. However, full 3D inversion is computationally intensive, requiring weeks or months of computer time on very large optimized systems. See PCT Patent Publication No. WO03/025803; and Carazzone, et al., “Three Dimensional Imaging of Marine CSEM Data”, SEG (Society of Exploration Geophysicists) Annual Meeting Extended Abstracts (2005).
Approximate 3D inversion schemes can potentially be reasonably accurate and faster means to analyze MCSEM data. This approach includes three-dimensional inversions based on weak scattering approximations of electromagnetic integral equation modeling (M. S. Zhdanov, Geophysical Inverse Theory and Regularization Problems, Elsevier, Amsterdam—New York—Tokyo, 628 pp. (2002); and T. J. Cui, et al., “3D imaging of buried targets in very lossy earth by inversion of VETEM data,” IEEE Trans. Geoscience Remote Sensing 41, 2197-2210 (2003)). These methods may not be readily available for industrial MCSEM applications and their efficiency in multi-source surveying may need to be confirmed.
Another approach to analyzing MCSEM data is iterative 3D forward modeling. In this approach, realistic detailed 3D geologic models are built for the reservoir and surrounding areas using seismic, petrophysical, and other subsurface information. The models are then simulated using forward 3D electromagnetic modeling, and modified until results consistent with electromagnetic, seismic, petrophysical, and/or other subsurface data are achieved (Green et al., “R3M Case Studies: Detecting Reservoir Resistivity in Complex Settings”, SEG Annual Meeting Extended Abstracts (2005)). This approach can lead to an accurate definition of subsurface resistivity leading to direct hydrocarbon detection, but it is time-consuming and labor-intensive.
Another common approach to analyzing MCSEM data is 1D inversion of amplitude and/or phase of measured electric field data (Mittet, et al., “Inversion of SBL Data Acquired in Shallow Waters”, EAGE (European Association of Geoscientists and Engineers) 66th Conference & Exhibition (2004)). This method can quickly generate 1D resistivity depth profiles for every receiver. However, this method only resolves 1D resistivity effects; it omits the 3D effects, which frequently are the effects of interest when applying MCSEM to detection or characterization of hydrocarbon reservoirs. The present invention employs 1D inversion, but includes additional features to overcome some of the limitations of this technique.
Another method to visualize and interpret MCSEM data is to normalize the recorded field data by a background electric field response (Ellingsrud et al., “Remote sensing of hydrocarbon layers by seabed logging (SBL): results from a cruise offshore Angola,” The Leading Edge 21, 972-982 (2002)). Maps based on ratios of observed amplitudes and background values at a constant offset were generated. The background response is measured electric field data from a receiver away from the target.
Among non-CSEM techniques, U.S. Pat. No. 6,098,019 to Hakvoort et al. discloses a method for determining electrical resistivity of an earth formation in the vicinity of a wellbore from measurements made by a resistivity logging tool, taking into account invasion of wellbore fluid into the formation. Resistivities at varying radical distances are determined by inversion of the logging data, which is done by updating modeled resistivity profiles in an iterative manner. Updating in a given radial interval is done using a ratio of the resistivity log to the modeled log for that radial interval. The method results in a layered 2D model of resistivity as a radially symmetric function of r and z, where the z-dependence is directly provided by placement of the tool at various depths in the wellbore.
There is another approach based on normalized electric field responses to visualize and interpret MCSEM data (K. E. Green et al., “Method for identifying resistivity anomalies in electromagnetic survey data”, U.S. Patent Publication No. US 2006/0197534). The data are displayed as amplitudes relative to background at all offsets along a towline, generating relative amplitude sections. This technique is called RASCAL by its inventors.
A method was developed by Burtz, et al., to interpret electromagnetic data using a layer-striping approach. The method begins by using high frequency data to model the shallow resistivity structure of the earth, and gradually models the earth to increasing depths by matching lower frequency data (PCT Patent Publication No. WO2006/096328). An embodiment of the present invention referred to herein as Shallow Background Model mapping is an alternative means of mapping resistive anomalies below shallow resistive bodies. It is a rapid approach, which can be done without reference to seismic, borehole, or other conventional subsurface data, and does not rely on iterative 3D forward modeling or full 3D inversion.